High power density is an important developing trend for power electronic products, especially for communication power supply products. A half-bridge LLC resonance DC/DC topology has a prominent advantage in terms of conversion efficiency and power density for medium and low power levels, therefore, it is widespread in telecommunication power supply products. With the development of the products, the degree of difficulty to improve the power density becomes greater. Reducing the number of components is a preliminary means other than production of new components and new arts.
As shown in FIG. 1, the industry depends on a primary side peak circuit sampling and a limited current circuit to complete a half-bridge LLC resonance DC/DC converter. The solution has advantages such as simpleness, quickness and high dependability. However, to meet the requirement of high power density, an extra primary side current sampling unit (normally it is implemented by a current transformer) occupies a lot of spaces. In addition, when safety distance is taken into consideration, certain difficulty is brought to the PCB layout and sketch pad.